1. Field of the Invention
This invention relates to a caster and particularly to a caster mounted on the underside of a vehicle, e.g. a forklift.
2. Description of the Prior Art
Common prior art casters mounted on vehicles swing through 180.degree. when the vehicle body reverses its direction of advance.
For example, Japanese published unexamined utility model application No. 55-13123 discloses a caster as shown in FIG. 6. This caster comprises an annular ball bearing race plate 100 fixed to the underside of the vehicle body, a journal plate 101 coupling with the fixed race plate 100 via a plurality of balls to form a ball bearing 102, and a wheel 103 having an axle 104 journalled within the lower end of the journal plate 101. The ball bearing 102 does not allow the journal plate 101 to move radially with respect to the fixed race plate 100. The point of contact of the tread 105 of the wheel 103 with the ground is offset by a distance t.sub.o from the axis of rotation 106 of the ball bearing 102. Thus, as the caster-mounted vehicle moves, the distance t.sub.o causes the wheel 103 to trail. In particular, when the caster-mounted vehicle reverses its direction of advance, the wheel 103 swings through 180.degree. on its tread 105 due to the load on the wheel 103 transmitted between the tread 105 and the ground. This swinging motion of the wheel 103 causes a jerk, i.e. a force causing irregular movement of the caster-mounted vehicle.
However, Japanese published unexamined utility model application No. 55-117302 discloses a caster in which the axis of rotation of the wheel can shift horizontally relative to the axis of the ball bearing when the caster-mounted vehicle reverses its direction of advance. This caster comprises a wheel, rectangular inner frameworks retaining the opposite ends of the axle of the wheel, a plurality of rollers mounted on the upper and lower edges of the inner frameworks, rectangular outer frameworks, along the inner surfaces of which the rollers rolls, arms supporting the outer frameworks, and a turning ring mounting the arms onto the vehicle body. This caster has a large number of parts and is not compact.